Maybe the gold is it? Packed with cells that notice dark and light and detect shadows. I like all theories on this. It is all interesting. Too coincidental to ignore that it falls on the location of parietal eye. Might serve multiple functions. I suspect that no matter the ultimate purpose, that it evolved from a useful parietal eye. Look up studies.There is proof that they do more than influence circadian rhythms in certain species. They pick up shadows and alert to overhead danger. Where it could have gone from there, who knows. I think it would be just as cool if it hypnotizes or confuses predators.

Wish I had a copy of the book Gerald suggests if it could shed some light.

For navigation around their home range, I'd guess. They appear to travel a whole lot more than mosquitofish do, from my observations. And being egg-layers, males might have established spawning sites (maybe several?) that they need to navigate to. Perhaps they have multiple spawning sites far apart based on sun exposure at different times of day??? Also, Fundulus are remarkably adept at responding quickly to overhead shadows/threats. I can corrall them with a big seine, but trying to catch them with a dipnet or 4x4 seines rarely works except when they're hiding in vegetation or leaf piles.

Lilyea- If it is in fact a parietal eye, then "sun compass" is one of they current theories. Why is it so developed in Fundulus though?

Gerald Pottern
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Hangin' on the Neuse
"Taxonomy is the diaper used to organize the mess of evolution into discrete packages" - M.Sandel

Excellent. I think you have sound theories there. But no matter what, it does make me want to go straight to question 2) why on backs? Centrarchid goes straight to question 2 and you brought it up. Something to it. Possibly it proved functional in other ways, and these fish will be gold all across the dorsal in 100k years. That was just an extreme example. It seems that it must fulfill another purpose as well.

I have kept Northern Studfish in an aquarium for 2 years now, along with 4-5 years of observation in the wild. The tops of their heads look like crystals and they have a gold stripe directly in front of the dorsal. In the wild, they move in fairly large groups of 20 or so fish until they spawn. When spawning, I see them pair up along sandy shorelines. I have never seen them using their pre-dorsal stripes to signal each other,in the wild or in the aquarium. I see them flaring out their throat-pouches,or blackening around their eyes and flexing their bodies and fins to communicate all the time. I have also observed them under UV light and the stripe disappears showing no patterns or anything.

Perhaps the scale structure on the head is also for protection from being exposed so much at the surface. Concerning the pre-dorsal stripe, I know some minerals glow or show hidden patterns when exposed to UV light, but the fish did not react in that way. I conclusion, I have discovered that fish don't behave like rocks.🤓😝🙃😅😂